The right-back makes his first return to Old Trafford tomorrow since going to Newcastle on loan two years ago.
He goes back as a permanent Toon player, with his side in the top four and looking to shock United.
Simpson, 24, said: "I always have something to prove to Sir Alex.
"I'm a United fan. I had a good time there but I'm part of a good Newcastle team now.
"I go back to Old Trafford looking to play well and to prove a point."